1. Field of the Invention
This invention relates to automatic gain control circuits. More specifically, the present invention relates to apparatus and methods for digital automatic gain control for image systems.
2. Description of the Prior Art
Due to the advent of multimedia communications, the need has arisen for low-cost solid state imagers to complement communication devices and computers. The image input device is an integral part of any teleconferencing and multimedia application. An important advantage of complementary metal-oxide semiconductor (CMOS) image sensors is that signal processing circuits can be readily integrated on the same chip as the imager, thus enabling the design of smart single-chip camera systems. CMOS imagers are inherently lower cost than conventional charge-coupled devices (CCDs) because the CMOS imagers can be manufactured in conventional, widespread CMOS fabrication lines without any process modification.
Present day electronic cameras employ automatic gain control (AGC) circuits that dynamically adjust the amplification gain (i.e., dynamic range) of the light-induced electrical signal so that a relatively "dark" picture appears "bright" to the user. The automatic gain control circuits are implemented with analog circuits because the light-induced electrical signal is analog. With a CMOS imager sensor, it is possible to incorporate the analog-to-digital conversion function on the same chip as the image sensor. Accordingly, the light-induced electrical signal can be converted into the corresponding digital signal early in the signal processing chain. This technique is disclosed in co-pending U.S. patent application Ser. No. 08/876,694, entitled "Image Sensor with Direct Digital Correlated Double Sampling", filed on Jun. 12, 1997, assigned to the assignee herein, incorporated by reference herein. Therefore, it is desirable to implement automatic gain control circuits using entirely digital means. It is to be appreciated that eliminating analog circuits from the system will make circuit design easier, and more compatible with conventional digital CMOS fabrication technologies.